The present invention relates to a pump motor.
Demands for safety and functionality of an automobile have become diversified and various mechanisms have thus been developed.
However, the space for installing a mechanism in an automobile is limited since space is enclosed in an automobile. Thus, each mechanism must be miniaturized to increase the number of mechanisms installed on the vehicle body. It is also required that motors functioning as actuators for the mechanisms also be miniaturized.
For instance, a motor for use in an anti-lock brake system includes an output shaft, which functions as a rotation shaft. The motor rotation shaft drives a piston pump that moves in a radial direction of the motor. It is required that the output shaft be shortened so that it does not project out of a pump housing in the axial direction of the motor.
Japanese Laid-Open Patent Publication No. 10-248225 describes a DC motor that shortens the output shaft. A commutator is arranged at the radially inner side of a core of an armature. Further, a brush is biased in the axial direction. This shortens the output shaft for a length corresponding to the dimension of the commutator.
Normally, the maneuverability and the rotation balance of the motor become unsatisfactory as the rotor diameter increases. This hinders immediate hydraulic pressurization of the pump driven by the motor and causes noise when the motor produces rotation. Thus, a motor with a small rotor diameter is preferred.
However, miniaturization of the rotor in the radial direction is difficult with the DC motor described in the above publication since the commutator is arranged in the radially inner side of the armature.
Further, since the brush is biased in the axial direction of the output shaft, a mechanism for biasing the brush towards the commutator must be arranged adjacent to the armature in the axial direction. Thus, it is difficult to shorten the motor as a whole in the axial direction.